The invention relates to the preparation of N,Nxe2x80x2-carbonylbislactams. It relates in particular to an improved process for the preparation of the latter from lactams and phosgene.
Processes for the preparation of N,Nxe2x80x2-carbonylbislactams are not very numerous. Phosgene is used in most of them. The second reactant is then a lactam or one of its derivatives, such as an alkaline salt or a trimethylsilyl derivative. None of these processes is satisfactory.
Thus, when phosgene is reacted with the sodium salt of xcex5-caprolactam according to H. R. Meyer, as described in the summary CA 52: 11781e of 1956, only from 0.6 to 40% of N,Nxe2x80x2-carbonylbiscaprolactam is obtained.
According to another method, as described in the article CA 68: 104571, xcex5-caprolactam and phosgene are reacted in the presence of a non-protonic amine, such as triethylamine, at a temperature of 20xc2x0 C. and then to 40xc2x0 C. The yield on the laboratory scale would be slightly higher, approximately 60%. However, when this example is repeated, the yield obtained is only 40%. Furthermore, if larger amounts of reactants are used, for example suitable for a 50-liter reactor, the yield is then only 23%.
In U.S. Pat. No. 5,972,237, the same method for the preparation of N,Nxe2x80x2-carbonylbiscaprolactam is employed. The amine used is dimethylcyclohexylamine. The yield indicated is then only 46%.
According to another process (Polymer Journal, 1995, 27(5), pp. 449-450), first of all the N-trimethylsilyl derivative of xcex5-caprolactam is formed and then a portion of the latter is reacted with phosgene to form the carbamoyl chloride. The two intermediate compounds are subsequently reacted with one another to produce the carbonylbiscaprolactam. Three stages are consequently needed. The yield is not mentioned. When this procedure is repeated, a yield of 51% is found.
N,Nxe2x80x2-Carbonylbislactams are very useful compounds, in particular as cocatalysts and activators in the manufacture of polymers, such as polyamides or polyesters, as activators of inorganic peroxides in detergents or as intermediates in the synthesis of amino acids. For most of their applications, they have to be of high purity.
There consequently existed a need to produce N,Nxe2x80x2-carbonylbislactams with a good yield, not only in the laboratory but also on an industrial scale, and under economical conditions.
Another object of the present invention is to obtain them with high purity.
The process according to the invention solves the problems indicated above. It consists in reacting phosgene with at least one lactam in the presence of a tertiary amine chosen from the group consisting of non-nucleophilic aliphatic tertiary amines.
N,Nxe2x80x2-Carbonylbislactams are obtained by virtue of this process generally with a yield of greater than 70% and a purity of greater than 99%.
The process is particularly advantageous for converting lactams of formula (I): 
in which n represents an integer from 3 to 15 and preferably from 5 to 12.
The N,Nxe2x80x2-carbonylbislactams obtained have the formula (II): 
When a single lactam is used, both rings of the compound are identical. When several different lactams are used as starting compounds, a mixture of N,Nxe2x80x2-carbonylbislactams with identical or different rings is obtained.
Phosgene is reacted with the lactam or lactams generally in a stoichiometric amount or an amount close to this value, i.e. approximately 0.5 mol of phosgene per mole of lactam. A large molar excess of phosgene is not necessary. Preferably, from 0.5 to 0.55 mol of phosgene is used per mole of lactams.
In a particularly unexpected way, it has been found that the nature of the amine was very important in solving the problems which were posed in the production of N,Nxe2x80x2-carbonylbislactams under good conditions.
The amine has to be selected from aliphatic tertiary amines which are non-nucleophilic or virtually non-nucleophilic, that is to say which are unreactive or virtually unreactive with respect to electrophilic entities, such as, for example, the carbonyl group. These amines are generally amines having very bulky radicals which thus prevent the free electron doublet of nitrogen from being accessible.
It is preferable for the amines also to be highly basic.
The useful amines are in particular amines of formula NR1R2R3 in which R1 represents the methyl or ethyl radical while R2 and R3, which are identical or different, represent the isopropyl or isobutyl radical. Mention may be made, as examples of such amines, of diisopropylmethylamine, diisopropylethylamine or diisobutylethylamine. Use is preferably made of diisopropylethylamine.
The presence of an amount of amine sufficient to trap the hydrochloric acid which is formed during the reaction is necessary. Use is generally made of approximately 1 mol of amine per mole of lactam and preferably of 0.95 to 1.05 mol per mole of lactam. An excess of amine is pointless.
According to the abovementioned processes of the prior art in which phosgene is used, the latter is introduced into the reaction medium at a temperature of approximately 20xc2x0 C. It has now been found that the process according to the invention is further improved by introducing the phosgene into the reaction medium at a temperature of between approximately xe2x88x9210xc2x0 C. and +5xc2x0 C. and preferably of between approximately xe2x88x925xc2x0 C. and 0xc2x0 C. The yields are better and the purity even higher. The introduction of phosgene generally lasts one to several hours.
The reaction is generally carried out in an organic solvent which is inert with respect to the reactants and which has a melting point below the temperature for introduction of the phosgene, in particular below approximately xe2x88x9210xc2x0 C. The boiling point of the solvent is preferably less than approximately 160xc2x0 C. Aromatic solvents, such as toluene and xylene, are highly suitable.
When all the amount of phosgene has been introduced, the reaction is allowed to continue at a temperature of approximately 40xc2x0 to 50xc2x0 C., generally for a few hours.
To recover the N,Nxe2x80x2-carbonylbislactams obtained, the amine hydrochloride which is formed is generally separated from the medium, for example by filtration or by washing with water followed by the recovery of the aqueous phase by separation by settling, and then the solvent is removed, in particular by evaporation under reduced pressure. A compound which is not a solvent for carbonylbislactams, such as isopropanol, methanol or water, can optionally then be added to the medium and then the carbonylbislactams are precipitated. If necessary, they can furthermore be purified by washing with an alcohol, such as, for example, methanol, and/or a recystallization can be carried out.
The yields obtained by virtue of the process of the invention are markedly improved with respect to those of the prior art. Thus, N,Nxe2x80x2-carbonylbiscaprolactam can be obtained with a yield in the laboratory of greater than 80% and on the industrial scale of greater than 70%. The purity is excellent and generally greater than 99%.
The process according to the invention is particularly economical because a means has also been found for recovering the diisopropylethylamine with a high purity from its hydrochloride formed during the reaction. To do this, an aqueous solution of the amine hydrochloride with water, preferably very pure water, is formed, either after having separated the hydrochloride from the reaction medium or by adding water to the medium and by recovering the aqueous phase, for example by separating by settling, and then this solution is neutralized with an inorganic alkaline base, such as lithium hydroxide, sodium hydroxide or potassium hydroxide, in particular in aqueous solution. Sodium hydroxide is the preferred base.
Use is generally made of an amount of base such that the final pH of the solution is approximately 13.
A codistillation of the solution is then carried out, generally at a temperature of between approximately 83xc2x0 C. and approximately 91xc2x0 C. at standard pressure. The amine is separated by settling from the mixture recovered. It generally has a purity, determined by GC analysis, equal to or greater than 99% and can be used in a further cycle for the production of N,Nxe2x80x2-carbonylbislactams.